1. Field of the Invention
This invention relates to computer disk drives, and more particularly to a computer disk drive system and associated power supply for the drive motor and head positioner circuitry.
2. Background of the Invention
Digital data may be stored on flexible or "floppy" disks or on hard or Winchester-type disks by the magnetization of successive small areas on the magnetic surface of the disk, by means of a magnetic head as the disk rotates. The density of digital storage on hard disk memory systems is on the order of 10 to 20 times the density achieved with floppy disk memory systems.
In the field of hard disk systems, sometimes referred to as Winchester-type disk systems, the rigid disk are normally formed of an aluminum alloy, and have a magnetizable coating on their upper and lower surfaces.
Rigid magnetic storage disks were originally relatively large in diameter, but in the last few years, the size has been reduced to that 51/4 inch disk systems are now widely available. Further, industry standard dimensions have been established for 51/4 inch Winchester-type disk units, and these dimensions include a height of 3.25 inches (82.6 millimeters), a width of 5.75 inches (146 millimeters), and a depth of 8.00 inches (203 millimeters). Environmental standards have also been established, with the operating temperature extending from 5.degree. C. to 50.degree. C. and the non-operating or storage temperature extending from -40.degree. C. to +65.degree. C.
Hard disk drives, especially of the Winchester-type, have continually been improved in design to allow the storage of progressively more data. Recent series disk drives provide in excess of 380M bytes of data in a standard 51/4 inch disk system, where "M bytes" stands for millions of bytes of digitial storage, and each byte contains eight binary digits or bits of data. High performance positioning systems allow average access time of 18 msec (milliseconds). In order to conserve space while allowing more storage room for additional disks, the drive motor has been incorporated into the spindle hub to make more vertical space available for the storage platters.
A preferred motor is a spindle-mounted four-pole delta-wound D.C. brushless motor with an armature resistance of one ohm and a torque constant of 2.6 oz-in/amp. The power dissipation within the drive must be limited to 20 watts while idling and 29 watts while seeking. Due to space considerations and the fact that the motor is mounted inside the spindle hub, this larger capacity disk drive must be capable of operating in an ambient temperature of 50.degree. C. (122.degree. F.), with no provisions for a cooling fan, while keeping the temperature within the drive below 65.degree. C. (149.degree. F.). This is important because the media as well as various electronic components are industry rated not much above 65.degree. C.
In order to run the disk drive within the above-stated parameters, the current drawn from the 12 V power supply will have to be limited. This requires limiting the motor current, for example, to 3 amperes during the start-up of the motor and 2 amperes during normal operation of the disk drive. Also it is desirable to reduce the current ripple in the +12 V line as many power supplies today use switching regulators which do not operate properly in the presence of fluctuating loads. Additionally for the given amount, of average power dissipation within the drive, reducing the current ripple in the power line also reduces the peak power requirement for the power supply.
Power supply methods for Winchester hard disk drives usually envision a linear voltage regulator for speed control. Linear regulators always have substantial power dissipation in the electronics. On motor start-up, even when the mechanical power required is small, the power drawn from the power supply is still very large. High power dissipation in the linear speed regulator results in high electronics temperatures which can not easily be handled where compact design considerations prohibit the use of a cooling fan.
In addition, the linear voltage regulators which have been used heretofore in disk drives, severely limit the initial power supplied to the motor, resulting in low acceleration rates, and consequent relatively long start-up times, before the unit is ready for information storage use.
An additional requirement for any speed control system will be to provide adequate braking and a rapid stop-start transition capability. A high initial torque is required at starts to overcome the "stiction-friction" phenomena which occurs where a magnetic head or slider at rest on a disk surface has high starting friction almost amounting to a "bond" between the head-disk interface. Once the disks are rotating at a moderate speed, the magnetic heads or sliders start "flying" with a thin cushion of air between the heads and the disk surface. At both stops and starts, it would be desirable for an improved winchester-drive to minimize the head-disk contact time. Mechanical braking is not feasible for a Winchester disk drive with the motor mounted inside the spindle hub. The dynamic braking which short circuits the windings via the drive transistors is the best practical way to provide the braking.
One prior art regulator for a variable speed motor, and which is not disclosed as involving disk drives, is U.S. Pat. No. 4,359,674 to Gotou. The Gotou patent shows a bi-polar switch controlling power through a diode, and an inductor and capacitor network, as shown in FIG. 6, box 52 of the U.S. Pat. No. 4,359,674 patent. Switching rates in such a regulator were at a relatively low frequency range. In such a system, the switching method is independent of the speed control and current limit factions. The duty cycle of the switching regulator is controlled so as to minimize the voltage drops across the power transistors to the predetermined small value yet keeping them in the linear operating ragne. It appears that the speed control function has been achieved by controlling them in the linear operating range. It appears that the speed control function has been achieved by controlling the current through the coils X, Y, Z (his FIG. 6) by means of command signal 60 of unspecified origin. Therefore the primary purpose of the switching regulator is to reduce the voltage applied to the motor (vc) to the minimum possible level consistent with still operating the drive transistors in the linear operating range.
Also, it may be noted that the switching regulator current switching by transistor 100 is directly reflected in the power line, so that there are output current pulses from the input DC voltage source, as there is no provision for smoothing these current fluctuations.
Further, with regard to packaging considerations, such prior art switching arrangements required large capacitors and inductors which would not fit within the limited space available in a Winchester disk drive system.
Accordingly, a principal object of the invention is to provide a low heat dissipation power supply which is compact enough to fit within the severely limited space of a Winchester-type disk drive; which will not overheat; and, which will provide maximum power to the motor, accelerating the disks until the heads are flying. Once running speed is obtained lesser levels of power will be provided, so that the power supply draws additional power only as needed.
A prime design objective is a power supply and control system which has high efficiency and minimal power dissipation in any of the electronics, conserving power to accelerate and rotate the disks.